Packaging laminate, method for its manufacturing and packaging container manufactured from the packaging laminate

ABSTRACT

A packaging laminate comprising a core layer of paper or carton with a through-going hole, a layer of thermoplastics applied onto one outer side of the core layer, an aluminium foil applied onto the other, inner, side of the core layer and bonded to the core layer by means of an intermediate layer of thermoplastics, the two layers of thermoplastics being sealed to each other within the region of the hole to form a membrane of aluminium foil and thermoplastics, and a first adhesive part-layer applied onto the inner side of the aluminium foil, a second intermediate part-layer of low density polyethylene applied adjacent to the first adhesive part-layer, and a third innermost part-layer comprising in the majority a metallocene polyethylene. The packaging laminate has improved sealability properties and provides improved or maintained package integrity and openability to a packaging container manufactured from the laminate, having an opening arrangement, which at opening removes the membrane of laminated layers of aluminium foil and thermoplastics from the region of the hole in the core layer.

TECHNICAL FIELD

The present invention relates to a packaging laminate comprising a corelayer of paper or carton with through-going holes, openings or slits, alayer of thermoplastics applied onto one outer side of the core layer,an aluminium foil applied onto the other, inner, side of the core layer,extending throughout the laminate, and bonded to the core layer by meansof an intermediate layer of thermoplastics, the two layers ofthermoplastics both extending throughout the laminate and being sealedto each other within the regions of the holes to form a membrane ofaluminium foil and thermoplastics, and one or more layers ofthermoplastics applied onto the other, inner, side of the aluminiumfoil.

BACKGROUND ART

Packaging containers of the single use disposable type for liquid foodsare often produced from a packaging material of the type mentionedabove. One such commonly occurring packaging container is marketed underthe trademark TETRA BRIK ASEPTIC® and is principally employed for liquidfoods such as milk, fruit juice etc. The packaging material in thisknown packaging container typically comprises a core layer of paper orpaperboard and outer, liquid-tight layers of thermoplastics. In order torender the packaging container light- and gas-tight, in particularoxygen gas-tight, for example for the purpose of aseptic packaging andpackaging of fruit juices, the material in these packaging containers isnormally provided with at least one additional layer, most commonly analuminium foil, which moreover renders the packaging materialthermosealable by inductive thermosealing which is a rapid and efficientsealing technique for obtaining mechanically strong, liquid- andgas-tight sealing joints or seams during the production of thecontainers.

Packaging containers are generally produced by means of modern packagingmachines of the type which form, fill and seal packages from a web orfrom prefabricated blanks of packaging material. From, for example, aweb, packaging containers are produced in that the web is reformed intoa tube by both of the longitudinal edges of the web being united to oneanother in an overlap joint. The tube is filled with the intended liquidfood product and is divided into individual packages by repeatedtransversal seals of the tube at a distance from one another below thelevel of the contents in the tube. The packages are separated from thetube by incisions in the transverse seals and are given the desiredgeometric configuration, normally parallelepipedic, by fold formationalong prepared crease lines in the packaging material.

In the prior art, packaging laminates for this kind of packages usuallyhave an innermost, inside thermoplastic heat sealing layer, mostcommonly of a low density polyethylene (LDPE), which normally hasadequate properties for heat sealing and for the function of moisturebarrier towards the filled liquid contents of the package.

By innermost or inside layer is meant a layer which is applied on theside of the packaging laminate facing towards the inside of a packagingcontainer formed from the laminate, and which will be in contact withthe filled contents of a filled packaging container.

From the consumer's point of view, it is desirable that the packagingcontainer be easy to handle and easy to open when it is time to emptythe package of its contents, and in order to satisfy this need, thepackaging container is often provided with some type of openingarrangement, with the aid of which it may readily be opened without theneed to employ scissors or other implements.

A commonly occurring opening arrangement in such packaging containersincludes a hole punched in the core layer of the package wall, the holebeing covered on the inside and outside of the package wall, by therespective outer layers of the packaging wall which are sealed to oneanother in the region of the opening contour of the through-going hole,thus forming a membrane of the layers not being paperboard. One exampleof a prior art opening arrangement has a separate pull-tab or openingstrip which is applied over the hole and which is rupturably sealed tothe outer layer of the outside of the package wall along a sealing jointaround the entire opening contour of the hole and at the same timepermanently sealed to the outer layer in the region inside the openingcontour of the hole. In more advanced opening arrangements, an openingdevice, usually of moulded plastics, having a pouring spout and a screwtop for resealing, is applied onto the region of and around the hole,which opening device is designed to penetrate or remove the membranewithin the hole region by a pushing-down or screwing-down movement or,alternatively, to remove the membrane by a screwing- and/or pulling-upmovement of the opening device. In the latter kind of opening device,the inside of a screwable part of the opening device is adhered to themembrane of the hole, in such a way that when it is screwed upwards awayfrom the packaging wall, the membrane is lifted along with the screwablepart and torn away from the edges of the hole, leaving a practicallyclean-cut hole for pouring the filled contents out of the package.

In particular, the latter kind of opening arrangement may functionsimilarly to a screw top of a bottle and is often desirable, since itavoids pushing residues of the membrane down through the hole into thepackage and the filled product.

A precondition for such an opening arrangement to function efficientlyand expediently is that, there is adequate adhesion between thedifferent layers of the membrane such that it does not delaminate whenscrewing- and/or pulling-up forces, or forces of screwing and/orpushing-down, are applied to it during the opening operation.

It is generally difficult to obtain such adequate adhesion within theregions of the holes, because of the difference in total laminatethickness between the regions of the holes and the regions outside ofthe holes when laminating together the aluminium foil and thermoplasticlayers of the membrane. When passing a web of the laminated layersthrough a press nip in a lamination station, the layers are pressed toadhere to each other by means of a pressure roller and a cooledcylinder. In the regions defined by the hole or the slit, the press nipis unable to press the aluminium foil and the polymer layers togethersufficiently for achieving the requisite adherence.

Thus, the thickness variations of the core layer may cause that thealuminium foil, which is relatively thin, will not be pressed againstand adhered sufficiently well to the surrounding layers ofthermoplastics within the whole of the region defined by the hole, whichmeans that air may be entrapped adjacent to he edges of the holes. Thisin turn means that there may be fracture formations in the aluminiumfoil, which may lead to the gas-tightness of the packaging containerbeing impaired and thereby also the colour, taste and nutritional valuesof the packaged food product. Furthermore, the integrity of the packagemay be impaired, which in turn may disturb the aseptic performance ofthe package.

The air inclusions also result in it being difficult to tear off orpenetrate the membrane consisting of the aluminium foil and thepolymeric films in the hole or slit, with the ability to open thepackaging being restricted and/or with it not being possible to make aclean cut when penetrating, resulting in the formation of frayed edges.

These problems have hitherto been eliminated or at least reduced to anacceptable level, by means of a press roller comprising a metal corewith a circular-cylindrical jacket surface, which jacket surface isfaced with an inner facing layer consisting of an elastic material,having a first hardness and a first thickness, and arranged on theoutside of the inner facing layer, an outer facing layer consisting ofan elastic material, having a second hardness and a a second thickness,with the first hardness is greater than the second hardness and with thefirst thickness being greater than the second thickness. Preferably, thefirst hardness being at least 15% greater, more preferably 20% and mostpreferably 25% greater, than the second hardness, calculated in Shore A,with the outer facing layer exhibiting a hardness of 50-80 Shore A,preferably 60-75 Shore A. Preferably, the second thickness constitutes5-25%, more preferably 7-20% and most preferably 8-15%, of the total ofthe first thickness and the second thickness. Preferably, the secondthickness is 1-10 mm, more preferably 1-5 mm and most preferably 1-3 mm.

Such a press roller has been described in a separate pending applicationWO 01/02751, also belonging to the applicant of the present application.

Due to the outer facing layer of lower hardness, a desired penetrationis achieved in the hole regions in the core layer, when the core layer,the aluminium foil and the polymeric layers pass through the press nip,at the same time as the low thickness of the outer, softer facing layerresults in the press nip length not being appreciably extended, meaningthat a desired pressure can be maintained in the press nip while stillretaining a high line load. The inner and outer facing layers may bemade of the same or different elastomeric materials, such as for examplerubber or polyurethane materials.

The described press roller should be used in at least one of thelamination station for the inside layers and/or the lamination stationfor the aluminium foil and the intermediate bonding layer. It may alsobe used in the lamination of the thermoplastic layer on the outside ofthe core paperboard layer.

For the purpose of an opening arrangement as described above, inparticular the one functioning by a screw-pulling motion, an innermostheat sealing layer of LDPE usually is bonded to the aluminium foil bymeans of a bonding layer of an adhesive polymer, such as for example agraft modified polyolefin or a copolymer of ethylene and (meth)acrylicacid or a ionomer.

During the latest years, there has been an increasing interest for theuse of innermost layers in packaging laminates comprising the kind ofethylene-alfa-olefin copolymers that are polymerized in the presence ofa metallocene catalyst, i.e. metallocene polyethylenes (hereinafterdenoted as m-PE), which normally are a type of linear low densitypolyethylenes (m-LLDPE).

Metallocene-polymerised polyethylenes generally have desirableproperties like improved tear and puncture resistance, toughness, impactstrength, clarity, antiblocking properties and heat sealing performancecompared to that of ordinary LDPE. In the manufacturing of packagingcontainers it would therefore be highly desirable to be able to use m-PEin the innermost sealing layer in order to improve package integrity andsealability properties.

With package integrity is generally meant the package durability, i.e.the resistance to leakage of the packaging container. This is tested ina first step by measuring the electrical conductivity through thepackaging laminate of the package in order to indicate if there is anykind of hole or crack in the inside thermoplastic layer(s). In a secondstep, the size and shape of the hole or crack is studied further byimmersing the packaging laminate into a red ink solution, whereby thepaperboard core layer will be coloured in red at and around the hole orcrack. The result is reported in number of containers with leakages outof 3000 tested packages. However, the test result does not automaticallymean that the containers would actually be leaking out the filledcontents, because the test method is more severe and shows very finecracks and holes that would perhaps not normally cause problems in real,daily storage and use. In general, it has been seen that by using m-PEin the innermost sealing layer instead of LDPE, the package integrity isat least equally reliable even when employing thinner layers of the m-PEthan the LDPE.

With sealability properties are meant the ability to heat sealappropriately within a temperature interval or interval of power supply.There, are for example three heat seals in a normal parallellepipedicpackaging container of the TETRA BRIK ASEPTIC® type, i.e. the sealtransversally of the tube, the longitudinal overlapping seal along thetube and the seal of the longitudinal strip over and along thelongitudinal seal on the inside of the tube. The transversal sealinvolves the double thickness of the packaging laminate and demands thehighest power for sealing. It has generally been seen that the “window”or interval of temperatures and power supply within which adequatesealing is performed is greater for an m-PE than for normal LDPE heatsealing inside layers. For example, in a TETRA BRIK ASEPTIC® fillingmachine, (such as TBA/8), which seals the packages by inductive sealing,the power supply to the inductive heating unit, is measured by scaleunits. The result of the test is reported as the range of scale units,within which adequate sealing is performed. Thus, for an m-PE insidematerial that is comparable to an equivalent LDPE inside material, therange of reported scale units for the transversal seal is much wider.This means that it will be less critical to exactly adjust thetemperature settings of the sealing part of the filing machines, whichare mostly operated by personnel at diaries and packaging plants, andthat the sealing operation will be more reliable and less sensitive tofluctuations in the temperature of the sealing tools.

Thus, by exchanging the innermost layer of thermoplastics in theabove-described packaging laminate from the traditional LDPE to a layercomprising an m-PE in the majority, heat sealability properties as wellas package integrity may be improved or, alternatively, maintained atlower amounts of heat sealable polymer.

However, when employing m-PE in the innermost layer of a conventionalpackaging laminate, instead of the normal LDPE, a considerabledeterioration in openability of the opening device appears. Suddenly themembrane breaks between the aluminium foil and the innermost layer ofthermoplastics, i.e. between the aluminium foil and the layer comprisingan adhesive polymer, due to the screwing and/or pulling movement of themembrane when opening the opening device, such that part of the membraneremains, covering the hole, and impedes the pouring-out of the contentsof the package. The problem appears to be caused by insufficientadhesion between the aluminium foil and the adjacent layer of adhesivepolymer on the inside of the aluminium foil.

DISCLOSURE OF THE INVENTION

It is, therefore, an object of the present invention to overcome oralleviate the above-described problems.

One object of the invention is to provide a packaging laminate withimproved sealability, having a core layer with punched holes, openingsor slits, which provides good openability of a packaging containerprovided with an opening device, which is arranged to remove themembrane of laminated layers of aluminium foil and thermoplastics fromthe region of the punched hole in the core layer at the openingoperation.

A further object of the invention is to provide a packaging laminatehaving improved sealability properties for the purpose of manufacturingan aseptic, gas-tight packaging container, having improved or maintainedpackage integrity, the packaging container being provided with anopening device, which is arranged to remove the membrane of laminatedlayers of aluminium foil and thermoplastics from the region of thepunched hole in the core layer at the opening operation.

These objects are attained according to the present invention by thepackaging laminate according to an exemplary aspect. Preferred andadvantageous embodiments of the packaging laminate according to thepresent invention have further been given the characterizing featuresaccording to additional exemplary aspects.

These objects are thus attained according to the present invention by apackaging laminate comprising a core layer of paper or carton withthrough-going holes, openings or slits, a layer of thermoplasticsapplied onto one outer side of the core layer, an aluminium foil appliedonto the other, inner, side of the core layer, extending throughout thelaminate, and bonded to the core layer by means of an intermediate layerof thermoplastics, the two layers of thermoplastics both extendingthroughout the laminate and being sealed to each other within theregions of the holes to form a membrane of aluminium foil andthermoplastics, and one or more layers of thermoplastic materialsapplied onto the other, inner, side of the aluminium foil, wherein oneor more layers of thermoplastic materials consists of three part-layers,being a first adhesive part-layer applied onto the aluminium foil, asecond intermediate part-layer of low density polyethylene (LDPE), and athird innermost part-layer comprising in the majority anethylene-α-olefin copolymer, polymerised in the presence of ametallocene catalyst, a so called metallocene polyethylene (m-PE).

By such a packaging laminate, having an inside structure comprisingthree part-layers respectively comprising adhesive polymer, LDPE andm-PE in this order, an improvement in sealability and package integrityqualities is obtained as well as required openability properties, forthe purpose of a packaging container having an opening arrangement,which at opening removes the membrane of laminated layers of aluminiumfoil and thermoplastics from the region of a hole in the core layer,made previous to lamination.

An improvement in sealability and package integrity properties wasexpected from the usage of an m-PE instead of an LDPE in the innermostlayer of a package in general, because these properties of m-PEqualities are known to a person skilled in the art. By improvement is tobe understood also when the amount of heat sealable polymer may bereduced at maintained package integrity properties.

However, by using m-PE in the inside layer, the above definedopenability was considerably deteriorated compared to when using LDPE,and it is believed that the strength of the m-PE became too high atopening, such that the weakest point at opening would be at theinterface between the aluminium foil and the layer of adhesive polymer,why delamination between these layers would occur instead of the entireremoval of the hole membrane.

According to a comparative example, wherein the thermoplastic structureinside of the aluminium foil consists of a first layer of an adhesivepolymer and a second innermost layer of either LDPE or m-PE, the m-PEvariant left clearly more residues of the membrane within the region ofthe hole, while the LDPE variant resulted in a good clean-cut openedhole.

Gradually, the inventors thus developed a theory that it is important toassure that the weakest point of the membrane at opening will not be atan interface between the layers of the membrane. The force needed tobreak loose the membrane from the packaging laminate around the edges ofthe hole, should not be higher than the adhesion force between thelayers, especially between the adhesive layer and the aluminium foil,since this has often shown to be the weakest point during thedevelopment work in connection with this invention. In order to move theweakest point away from this interface between layers, mainly twoconsiderations are believed to be of importance.

Firstly, the adhesion force between the aluminium foil and the layer ofadhesive polymer should be above a certain level, why it, for example,also has been seen that it is necessary to employ a layer of an adhesivepolymer for the contact with the aluminium foil, which measure however,not is sufficient in itself. The choice of adhesive polymer may play arole of course, in addition to other ways of increasing the adhesion.

Secondly, the force needed to tear the membrane away from the packaginglaminate around the edges of the hole should be balanced against theabove adhesion force, i.e. the strength of the sealing layer(LDPE/m-PE), along the plane of the layer in the packaging laminatearound the edge of the hole, should be below a certain level in relationto the adhesion strength between aluminium foil and adhesive layer.

Furthermore, these two considerations for openability should be balancedto match the requirements on improved sealing and package integrityproperties from the usage of m-PE in the sealing layer(s).

While investigating this theory, it was unexpectedly found that aco-extrusion coated combination of an innermost layer comprising m-PEwith an intermediate layer of LDPE and a layer of an adhesive polymerapplied towards the aluminium foil, enabled adequate openabilityproperties as described above, at the same time as improved sealabilityand improved or maintained package integrity properties were obtained.

In a test series a packaging laminate having a two-layer insidestructure was comparable to a packaging laminate having a three-layerinside structure.

The packaging laminates had the structure of an outer layer of about 16g/m² of LDPE, a paperboard core layer, an intermediate bonding layer ofabout 23 g/m² of an LDPE, an aluminium foil, and the inside structure.The two-layer inside structure had a layer of about 6 g/m² of anadhesive ethylene-acrylic acid copolymer applied onto the aluminium foiland an innermost layer comprising m-PE in a quantity of about 20 g/m²applied next to the adhesive layer. The three-layer inside structure hada layer of about 6 g/m² of an adhesive ethylene-acrylic acid copolymerapplied onto the aluminium foil, an intermediate layer of about 10 g/m²of LDPE and an innermost layer comprising m-PE in a quantity of about 10g/m² applied next to the intermediate layer. The results showed aremarkable improvement for the three-layer variant in openability of ascrew-top opening device of the kind that removes the membrane by acombined screwing-up and pulling-up motion. From the two-layer insidepackaging laminate, almost all the packages opened at room temperatureshowed delaminations between the aluminium foil and the adhesive layer.The result was about the same at refrigerator temperature. However, fromthe three-layer inside packaging laminate, significantly fewer packagesshowed delaminations between the aluminium foil and the adhesive layer(8 out of 50 at both ambient and refrigerator temperature). The resultsfrom this early test series have since been improved further byoptimizing the laminate structure and the lamination process.

The innermost layer according to the invention comprises in the majorityan ethylene-alpha-olefin copolymer, which has been polymerized in thepresence of a single-site catalyst, containing a metallocene transitioncompound. The ethylene-alpha-olefin copolymer (m-PE) is most commonly alinear-low density polyethylene, i.e. an m-LLDPE. The m-PE useful forthe invention has a density of from about 0.865 to about 0.935,preferably from about 0.890 to about 0.925, more preferably from about0.900 to about 0.915 g/cm³ and a melt (flow) index (MI) of from about 1to about 20, preferably from about 10 to about 20 g/10 min at 190° C.,2.16 kg (ASTM 1278). The copolymerised alpha-olefin has 4-8 carbonatoms, and is selected from the group of 1-butene, 1-methyl-1-pentene,1-hexene, 1-heptene or 1-octene, and is usually copolymerised in anamount of about 15 weight-% or lower.

Preferably, and in particular in the case of the m-PE being a linearpolyethylene, it is advantageous to blend the m-PE with an LDPE in orderto obtain the required processability properties, such as properties ofmelt elasticity, strain hardening and shear thinning. Preferably, theinnermost layer comprises a blend of an m-PE with from about 20 up toless than 50 weight-% of LDPE, most preferably with from about 30 toabout 40 weight-% LDPE. In general, a blend of m-PE and LDPE should bewithin these ranges, because a blend comprising more than 80 weight-% ofm-PE tends to become more difficult to manage in the melt extrusionprocess while a blend comprising less than 50 weight-% of m-PE tends tolose the advantageous properties that are connected with the use ofm-PE. An optimally working blend for the purpose of the packaginglaminate of the invention has an amount of from about 30 to about 40weight-% of LDPE.

Preferably, the outer layer of thermoplastics and the intermediatebonding layer of thermoplastics comprise mainly a low densitypolyethylene (LDPE), although it may be possible to use a blend of adifferent or modified polyolefin with similar properties in theselayers, for preparing a packaging laminate according to the invention.

The LDPE's used according to the invention have a density of above about0.910 g/cm³, preferably of from 0.912-925 and more preferably of fromabout 0.915-0.920, a melt (flow) index (MI) of about 1-25 g/10 min (ASTM1278) and are non-linear qualities, thus excluding LLDPE and m-LLDPE.

The LDPE of the second intermediate part-layer, the LDPE used in theouter layer on the other side of the core layer and the LDPE in theintermediate bonding layer between the core layer and the aluminiumfoil, may be of the same or of a different kind.

The innermost three-layer structure of the laminate should have acertain bulk total thickness of heat sealable ethylene polymer layers.However, it is sufficient with a thinner layer comprising the m-PE thanthe layer of LDPE, in order to provide the improved sealability andpackage integrity properties and, at the same time, good openabilityproperties.

Preferably, the second intermediate part-layer of LDPE is applied in aquantity of 10-25, preferably 16-20 g/m² and the innermost part-layercomprising m-PE is applied in a quantity of 8-18, preferably 8-12 g/m².The most preferable quantities of the respective layers are about 18g/m² of the LDPE and about 10 g/m² of the innermost layer comprising them-PE. It is to be understood that the variations of the polymerquantities applied by an extrusion coating process, normally lies withinabout +/−2 g/m².

Preferably, the total, bulk amount applied of the innermost three-layerstructure is at least about 32-38 g/m². At lower total amounts, it wouldbe necessary to apply higher amounts of the innermost part-layercomprising m-PE in the majority, in order to obtain an improvement inpackage integrity and sealability, which higher amounts would, however,affect the openability properties negatively. On the other hand at toohigh total amounts of the part-layers, openability-would still benegatively affected because of the resistance to tearing and pulling ofthe total bulk of layers. Thus, there has been found to be an optimumtotal amount at about 35 g/m².

The first part-layer comprising an adhesive polymer, preferably alsoshould have a certain thickness and contribute to the total thickness ofthe innermost three-layer structure. The thickness of the adhesivepart-layer may be from 1 to 10 g/m². An optimal, adhesive layerthickness was found at a quantity applied of about 4-9 g/m², mostpreferably at a quantity applied of about 7 g/m². Higher amounts appliedwould render the packaging laminate more expensive due to the generallyhigher costs of adhesive polymers.

Preferably, the first part-layer according to the invention comprises acopolymer of ethylene and (meth)acrylic acid (EAA or EMAA). Examples ofsuch polymers are traded under the name of Primacor, Escor and the like.Also other modified polyolefins are feasible as adhesive polymers forthe invention, e.g. maleic-anhydride modified polyethylenes, e.g. tradedunder the name of Bynel, but are less preferred. However, the(meth)acrylic-acid type of modified polyethylenes appear to give thebest adhesion results in the particular packaging laminate for thepurpose of the invention. A possible equal alternative could be ionomeradhesives of the type traded under the name of Surlyn.

The paperboard qualities intended for the packaging laminate of theinvention generally have a stiffness of from about 200 mN to about 400mN. In particular, 260 mN and 370 mN paperboards have been successfullytested.

According to another aspect of the invention, there is provided a methodof manufacturing the packaging laminate according to an exemplaryaspect, wherein the inner layer of thermoplastics is extrusion coated asthree part-layers, by applying a first adhesive part-layer onto thealuminium foil, a second intermediate part-layer of LDPE and a thirdinnermost part-layer comprising in the majority a m-PE. Preferred andadvantageous embodiments of the method according to the invention havethe characterizing features according to additional exemplary aspects.

Most preferably, the three part-layers are applied onto the other,inner, side of the aluminium foil by means of co-extrusion coating ofthe three layers in one process step. It would probably also be possibleto extrude the first and second part-layers together, while the thininnermost part-layer is extrusion coated onto the second intermediatepart-layer in a separate step. However, there are some importantadvantages with the co-extrusion coating of all three layerssimultaneously. Besides the obvious advantages of process simplicity,i.e. one extrusion coating process step instead of two, and increasedstability of the extruded melt film by the respective part-layerssupporting each other, there is also an important advantage in that thetemperature of the extruded melts will be kept higher until the meltfilm reaches the substrate aluminium foil, which in turn appears toimprove the adhesion between the adhesive part-layer and the foil. It isbelieved that the heat from the melt film of the second part-layer istransferred to the first adhesive part-layer, or the heat of the firstadhesive part-layer is better preserved during transport through the airgap due to the higher melt temperature of the intermediate part-layer.It was previously known that a polyethylene layer being extrusion coatedonto an aluminium foil would gain better adhesion towards the foil, thehigher the extrusion temperature is. It was, however, not known or evenexpected, that this effect would appear also via a layer of adhesivepolymer, which itself cannot be extruded at a too high temperaturebecause of the risk of degradation and a loss of adhesive propertiesinstead. By co-extruding the three part-layers all together, this effectof heat retention in the first part-layer will be as high as possibleand therefore that method is most preferable.

Preferably, the second intermediate part-layer is extruded at a highertemperature than the first and third part-layers. By doing so, heat willbe transferred from the second part-layer to the first adhesivepart-layer and the effect above will be increased further. By applying athicker second part-layer, even more high temperature heat will betransferred to the first adhesive part-layer. Preferably, the third,innermost, part-layer should be extruded at a temperature as low aspossible in order to avoid that possible trace substances from polymerdegradation, due to high temperature influence on the polymer melt, arereleased and migrating into the filled contents of the package, i.e. toavoid so called problems of “off-taste” in the filled food product.

Most preferably, the first part-layer comprising an adhesive polymer, aswell as the third part-layer comprising an m-PE in the majority, isextruded at the lower temperature of about 260-280° C., which is thetemperature of the melt in the extruder. This temperature does notdegrade the respective polymer melts and has proved to be an optimaltemperature for still obtaining good adhesion to the surrounding layers.The second intermediate part-layer of LDPE is co-extruded with the firstand third part-layers at a higher temperature of about 285-320° C.,preferably at about 310-315° C. At this temperature settings, animproved adhesion between the aluminium foil and the first adhesivelayer as well as improved openability, has been observed.

According to another preferred embodiment, the aluminium foil issubjected to a surface activation treatment by means of flame treatmentbefore it is coated with the three inner part-layers. Such treatmentwith gas flames against the aluminium foil surface, has proved toincrease the adhesion between the aluminium foil and the first adhesivepart-layer even further, and thus contributes to improved openabilityproperties of the membrane of the opening hole.

According to a further aspect of the invention, there is provided apackaging container manufactured from the packaging laminate of theinvention, having an optimized balance of openability properties againstpackage integrity and seal quality, according to exemplary aspects.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Further advantages and favourable characterizing features of the presentinvention will be apparent from the following detailed description, withreference to the appended figures, in which:

FIG. 1 is a cross-sectional view of a laminated packaging materialaccording to the present invention;

FIG. 2 schematically illustrates a method of manufacturing of thelaminated packaging material described in conjunction with FIG. 1;

FIG. 3 schematically illustrates an opening arrangement for which thepackaging laminate of the invention is particularly well suited; and

FIG. 4 is a side elevation in perspective of a configurationally stableand durable packaging container, equipped with an opening arrangement asexemplified in FIG. 3, which packaging container is produced from alaminated packaging material according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the packaging laminate of the invention 10 includesa core layer 11, which is a configurationally rigid but foldablepaperboard or carton. The core layer is provided with going-throughholes, opening or slits for the purpose of applying opening arrangementson the packages manufactured from the packaging laminate. On one side,being the outer side of a package produced from the laminate, i.e. theside facing outwards away from the filled food product, of the corelayer 11, a layer of heat sealable thermoplastics 12 is applied, whichalso provides a liquid and moisture barrier to that side of the corelayer. The outer thermoplastic layer is preferably applied in a quantityof about 10-17, preferably 12 g/m², and is in particular a polyethylene,such as a low density polyethylene (LDPE).

On the other, inner, side of the core layer, facing towards the insideof a package produced from the laminate, a gas barrier layer of analuminium foil 13 is applied by means of an intermediate bonding layerof thermoplastics 14, in particular a polyethylene, such as LDPE. Theintermediate bonding layer is applied in a quantity of from about 23 toabout 27, preferably about 25, g/m².

On the inner side of the aluminium foil, i.e. the side not bonded to thecore layer, a thermoplastic structure 15 of three part-layers isapplied. A first part-layer 15-1 comprising an adhesive polymer isarranged to contact the aluminium foil and provide sufficient adhesionbetween the aluminium foil and the second intermediate part-layer 15-2of LDPE, and a third innermost part-layer 15-3, comprising in themajority an ethylene-alpha-olefin, polymerised in the presence of asingle site, metallocene, catalyst (m-PE), is applied next to the secondpart-layer.

All of the layers 12, 13, 14, and 15-1,2,3 are extending throughout thelaminate and thus covering also the regions of the holes, on both sidesof the core layer.

The adhesive polymer is preferably a copolymer of ethylene and acrylicacid or methacrylic acid. The third innermost part-layer preferablyconsists of a blend of about 60-70 weight-% of m-PE and about 30-40weight-% of LDPE.

The most preferred quantities of the respective thermoplasticpart-layers are about 7 g/m² of the first part-layer 15-1, about 18 g/m²of the second part-layer 15-2 and about 10 g/m² of the third part-layer15-3. The inner structure of thermoplastic part-layers is thus appliedin a total quantity of about 35 g/m².

In a comparative example, one packaging laminate having an innerstructure of three part-layers in quantities of about 7, 18 and 10 g/m²as described above and a packaging laminate having an inner structurewith quantities of about 7, 14 and 14 g/m² were produced under the sameconditions. The 7-18-10 packaging laminate showed less failures atopening of packaging containers from the laminate, than the 7-14-14laminate, which in particular had a thicker third innermost part-layercomprising m-PE. With less failures is meant fewer delaminations of themembrane in the hole and fewer residues of polymer and aluminium foilfrom the membrane to be seen within the hole region.

Referring to FIG. 2, in a method 20 of manufacturing the packaginglaminate of the invention, a web of a paperboard core layer 21 is ledforward through a punching station 22, where the paperboard is providedwith holes for the purpose of later applied opening arrangements on thepackages. The holed web 21′ is further led forward to a laminationstation 25 wherein a web of aluminium foil 23 is laminated to one sideof the paperboard by extruding, from an extruder 24, an intermediatebonding layer of LDPE between the paperboard web and the web ofaluminium foil. The core layer and the aluminium foil are pressedtogether in a roller nip, thus, becoming a laminate of paperboard andaluminium foil 26. The laminate 26, is led forward to the nextlamination station 28, where the innermost structure of thermoplasticlayer part-layers 15 is applied by means of co-extrusion coating ontothe aluminium side of the laminate 26.

The three part-layers 15-1, 15-2 and 15-3, are fed from a respectiveextruder into a co-extrusion arrangement, preferably a feed block 27, tobe formed through a die into a three-layer melt film to be coated ontothe substrate, i.e. the aluminium side of the laminate 26, pressed andcooled in a roller nip 28, thus forming the laminate coated withheat-sealable thermoplastics on the inner side. The holed paperboardcore web 21′, may be coated with an outer layer of thermoplastics on itsother, outer, side, by extrusion coating (not shown) before the step oflamination to aluminium foil at 25, or after the step of extrusioncoating 28 of the innermost side part-layers, or between these processsteps.

A double-acting pressure roller may be used at all lamination stations25, 28 and at the lamination of the outer layer on the outside of thecore layer, in combination with a rather high nip pressure, in order toachieve optimal adhesion between the laminate layers of the membraneswithin the regions of the holes and avoiding air entrapments between thelayers close to the edges around the holes.

According to a preferred embodiment of the invention, the surface of thealuminium foil is activated by means of a flame treatment 29, before itis coated with the inside layers of thermoplastics 15, at 27.

In a comparative example, a packaging laminate according to theinvention with an innermost three part-layer structure, having about 7g/m² of the adhesive polymer, about 18 g/m² of the intermediate LDPE andabout 10 g/m² of the innermost part-layer comprising m-PE in themajority, was produced by extrusion coating with, and without, apreceding flame treatment of the aluminium foil inside. With precedingflame treatment, the adhesion force between the aluminium foil and thefirst adhesive part-layer was higher than without flame treatment. Inpackages manufactured from the laminate and equipped with thescrewing-up/pulling-up type of opening arrangements, there were very fewremarks on the openability, which moreover were merely aesthetical.Without the preceding flame treatment, there were some more residues ofthe membrane observed in the hole. Therefore, it has been concluded thata flame treatment of the aluminium foil is preferable when carrying outthe invention.

Referring to FIG. 3, in an example of an intended opening arrangement30, the laminated membrane 31, comprising layers of thermoplastics andaluminium foil, is sealed to a screw top 32, being part of an openingdevice applied to the packaging container, at an interface 33. Whenopening by screwing the screw top 32 upwards, by means of a threadarrangement with the surrounding parts of the opening device 34, themembrane is sheared along the edges around the hole 35 at the same timeas it is pulled upwards together with the top.

Referring to FIG. 4, a typical aseptic package of the type TETRA BRIKASEPTIC® is provided with an opening arrangement as described in FIG. 3.The hole with the membrane is provided at the top of the packagingcontainer and there is attached an opening device of moulded plasticsonto the membrane over the hole, which opening device consists of aframe and a screw top threaded into it. The opening device may befurther provided with a so called tampering evidence, the position ofwhich indicates if the packaging container has been opened or not.

By way of conclusion it should be observed that the present inventionwhich has been described above with particular reference to theaccompanying figures, is not restricted to these embodiments describedand shown exclusively by way of example, and that modifications andalterations obvious to a person skilled in the art are possible withoutdeparting from the inventive concept as disclosed in the appendedclaims.

1. Packaging laminate comprising a core layer of paper or carton with atleast one through-going hole, opening or slit, a layer of thermoplasticsapplied onto one outer side of the core layer, an aluminum foil appliedonto the other, inner, side of the core layer, extending throughout thelaminate and extending over the at least one through-going hole, openingor slit, and bonded to the core layer by an intermediate layer ofthermoplastics, the two layers of thermoplastics both extendingthroughout the laminate and being sealed to each other within the regionof the through-going hole, opening or slit, to form a membrane ofaluminum foil and thermoplastics, and one or more layers ofthermoplastic materials applied onto the other, inner, side of thealuminum foil, wherein, the one or more layers of thermoplasticmaterials consist of a first adhesive part-layer applied onto thealuminum foil, a second intermediate part-layer consisting of lowdensity polyethylene that is not a linear low density polyethylene, anda third innermost part-layer comprising in the majority a metallocenepolyethylene, which is an ethylene-α-olefin copolymer, polymerised inthe presence of a metallocene catalyst, wherein the first adhesivepart-layer is in direct contact with the aluminum foil, wherein thesecond intermediate part-layer is in direct contact with the firstadhesive-part layer, wherein the packaging laminate comprises an openingdevice comprising a screw top that is attached to the membrane ofaluminum foil and thermoplastics at the region of the through-goinghole, opening or slit, wherein the third innermost part-layer is theinnermost layer of the laminate and comprises the innermost surface ofthe laminate.
 2. Packaging laminate according to claim 1, wherein, thethird innermost part-layer is a blend of an m-PE with from about 20 tobelow 50 weight-% of LDPE.
 3. Packaging laminate according to claim 1,wherein, the third innermost part-layer is a blend of m-PE with fromabout 30 to about 40 weight-% of LDPE.
 4. Packaging laminate accordingto claim 1, wherein, the second intermediate part-layer is applied in aquantity of 16-20 g/m² and the innermost part-layer in a quantity of8-12 g/m².
 5. Packaging laminate according to claim 1, wherein, thesecond intermediate part-layer is applied in a quantity of about 18 g/m²and the innermost part-layer in a quantity of about 10 g/m². 6.Packaging laminate according to claim 1, wherein, the first adhesivepart-layer is applied in a quantity of about 4-9 g/m².
 7. Packaginglaminate according to claim 1, wherein, the three part layers areapplied in a total quantity of about 32-38 g/m².
 8. Packaging laminateaccording to claim 1, wherein, the adhesive part-layer comprises acopolymer of ethylene and (meth)acrylic acid.
 9. Packaging laminateaccording to claim 1, wherein, the thermoplastics used in the outerlayer and the intermediate bonding layer is an LDPE of the same ordifferent kind.
 10. Packaging container manufactured from the laminateas specified in claim
 1. 11. Packaging container according to claim 10,wherein the screw top is capable of opening the package by removing themembrane from the region by a combined screwing- and pulling-up motion.12. Packaging laminate to be formed into a packaging container,comprising: a core layer of paper or carton with at least onethrough-going hole, opening or slit, the core layer having a firstsurface and an oppositely positioned second surface; a first layer ofthermoplastic material applied to the first surface of the core layer toface towards an exterior when the packaging laminate is formed into thepackaging container; an aluminum foil bonded to the second surface ofthe core layer by a second layer of thermoplastics material, wherein thealuminum foil extends throughout the packaging laminate and over the atleast one through-going hole, opening or slit; the first and secondlayers of thermoplastic material extending throughout the packaginglaminate and being sealed to each other within the region of thethrough-going hole, opening or slit to form a membrane of aluminum foiland thermoplastics; a third layer of thermoplastic material applied to asurface of the aluminum foil facing away from the core layer; the thirdlayer of thermoplastic material comprising three part-layers; the threepart-layers forming the third layer comprising: a first adhesivepart-layer applied to the surface of the aluminum foil facing away fromthe core layer; a second intermediate part-layer consisting of lowdensity polyethylene that is not a linear low density polyethylene; athird innermost part-layer comprising a metallocene polyethylenepolymerised in the presence of a metallocene catalyst; the firstadhesive layer being positioned between and in direct contact with boththe aluminum foil and the second intermediate part-layer; and the secondintermediate part-layer being positioned between and in direct contactwith both the first adhesive part-layer and the third innermostpart-layer, wherein the third innermost part-layer is the innermostlayer of the laminate and comprises the innermost surface of thelaminate.
 13. Packaging container according to claim 12, wherein a screwtop is capable of opening the package by removing the membrane from theregion by a combined screwing- and pulling-up motion.
 14. Packaginglaminate comprising a core layer of paper or carton with at least onethrough-going hole, opening or slit, a layer of thermoplastics appliedonto one outer side of the core layer, an aluminum foil applied onto theother, inner, side of the core layer, extending throughout the laminateand extending over the at least one through-going hole, opening or slit,and bonded to the core layer by an intermediate layer of thermoplastics,the two layers of thermoplastics both extending throughout the laminateand being sealed to each other within the region of the through-goinghole, opening or slit, to form a membrane of aluminum foil andthermoplastics, and one or more layers of thermoplastic materialsapplied onto the other, inner, side of the aluminum foil, wherein theone or more layers of thermoplastic materials consist of a firstadhesive part-layer of a copolymer of ethylene and (meth)acrylic acidapplied onto the aluminum foil, a second intermediate part-layerconsisting of low density polyethylene that is not a linear low densitypolyethylene, and a third innermost part-layer comprising in themajority a metallocene polyethylene, which is an ethylene-α-olefincopolymer, polymerised in the presence of a metallocene catalyst,wherein the second intermediate part-layer is disposed between the firstadhesive part-layer and the third innermost part-layer, wherein thefirst adhesive part-layer is in direct contact with the aluminum foil,wherein the second intermediate part-layer is in direct contact with thefirst adhesive-part layer, wherein the third innermost part-layer is theinnermost layer of the laminate and comprises the innermost surface ofthe laminate.
 15. Packaging laminate according to claim 14, wherein, thethird innermost part-layer is a blend of an m-PE with from about 20 tobelow 50 weight-% of LDPE.
 16. Packaging laminate according to claim 14,wherein, the third innermost part-layer is a blend of m-PE with fromabout 30 to about 40 weight-% of LDPE.
 17. Packaging laminate accordingto claim 14, wherein, the second intermediate part-layer is applied in aquantity of 16-20 g/m² and the innermost part-layer in a quantity of8-12 g/m².
 18. Packaging laminate according to claim 14, wherein, thesecond intermediate part-layer is applied in a quantity of about 18 g/m²and the innermost part-layer in a quantity of about 10 g/m². 19.Packaging laminate according to claim 14, wherein, the first adhesivepart-layer is applied in a quantity of about 4-9 g/m².
 20. Packaginglaminate according to claim 14, wherein, the three part layers areapplied in a total quantity of about 32-38 g/m².
 21. Packaging laminateaccording to claim 14, wherein, the thermoplastics used in the outerlayer and the intermediate bonding layer is an LDPE of the same ordifferent kind.
 22. Packaging container manufactured from the laminateas specified in claim 14.